Add ItemAttributeModifierEvent (#7484)

patches/minecraft/net/minecraft/item/ItemStack.java.patch

@@ -214,7 +214,7 @@
        return list;
     }
  
-@@ -817,7 +841,7 @@
+@@ -817,9 +841,10 @@
              }
           }
        } else {
@@ -222,8 +222,11 @@
 +         multimap = this.func_77973_b().getAttributeModifiers(p_111283_1_, this);
        }
  
++      multimap = net.minecraftforge.common.ForgeHooks.getAttributeModifiers(this, p_111283_1_, multimap);
        return multimap;
-@@ -954,6 +978,35 @@
+    }
+ 
+@@ -954,6 +979,35 @@
        return this.func_77973_b().func_219971_r();
     }
  

src/main/java/net/minecraftforge/common/ForgeHooks.java

@@ -33,6 +33,7 @@ import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import com.google.common.base.Throwables;
 import com.google.common.collect.Lists;
+import com.google.common.collect.Multimap;
 import com.google.common.collect.Queues;
 import com.google.common.collect.Sets;
 import com.google.gson.Gson;
@@ -48,7 +49,10 @@ import it.unimi.dsi.fastutil.longs.LongSet;
 import net.minecraft.advancements.Advancement;
 import net.minecraft.block.Block;
 import net.minecraft.block.material.Material;
+import net.minecraft.entity.ai.attributes.Attribute;
+import net.minecraft.entity.ai.attributes.AttributeModifier;
 import net.minecraft.fluid.*;
+import net.minecraft.inventory.EquipmentSlotType;
 import net.minecraft.loot.LootContext;
 import net.minecraft.loot.LootTable;
 import net.minecraft.loot.LootTableManager;
@@ -125,6 +129,7 @@ import net.minecraftforge.common.world.MobSpawnInfoBuilder;
 import net.minecraftforge.event.AnvilUpdateEvent;
 import net.minecraftforge.event.DifficultyChangeEvent;
 import net.minecraftforge.event.ForgeEventFactory;
+import net.minecraftforge.event.ItemAttributeModifierEvent;
 import net.minecraftforge.event.ServerChatEvent;
 import net.minecraftforge.event.entity.EntityTravelToDimensionEvent;
 import net.minecraftforge.event.entity.item.ItemTossEvent;
@@ -1025,6 +1030,16 @@ public class ForgeHooks
         MinecraftForge.EVENT_BUS.post(new AdvancementEvent(player, advancement));
     }
 
+    /**
+     * Hook to fire {@link ItemAttributeModifierEvent}. Modders should use {@link ItemStack#getAttributeModifiers(EquipmentSlotType)} instead.
+     */
+    public static Multimap<Attribute,AttributeModifier> getAttributeModifiers(ItemStack stack, EquipmentSlotType equipmentSlot, Multimap<Attribute,AttributeModifier> attributes)
+    {
+        ItemAttributeModifierEvent event = new ItemAttributeModifierEvent(stack, equipmentSlot, attributes);
+        MinecraftForge.EVENT_BUS.post(event);
+        return event.getModifiers();
+    }
+
     /**
      * Used as the default implementation of {@link Item#getCreatorModId}. Call that method instead.
      */

src/main/java/net/minecraftforge/event/ItemAttributeModifierEvent.java

@@ -0,0 +1,143 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+package net.minecraftforge.event;
+
+import com.google.common.collect.HashMultimap;
+import com.google.common.collect.Multimaps;
+import net.minecraftforge.eventbus.api.Event;
+import net.minecraft.entity.ai.attributes.Attribute;
+import net.minecraft.entity.ai.attributes.AttributeModifier;
+import net.minecraft.inventory.EquipmentSlotType;
+import net.minecraft.item.ItemStack;
+import com.google.common.collect.Multimap;
+
+import javax.annotation.Nullable;
+import java.util.Collection;
+
+/**
+ * This event is fired when the attributes for an ItemStack are being calculated.
+ * Attributes are calculated on the server when equipping and unequipping an item to add and remove attributes respectively, both must be consistent.
+ * Attributes are calculated on the client when rendering an item's tooltip to show relevant attributes.
+ * <br>
+ * Note that this event is fired regardless of if the stack has NBT overriding attributes or not. If your attribute should be
+ * ignored when attributes are overridden, you can check for the presence of the {@code AttributeModifiers} tag.
+ * <br>
+ * This event is fired on the {@link net.minecraftforge.common.MinecraftForge#EVENT_BUS}.
+ */
+public class ItemAttributeModifierEvent extends Event
+{
+    private final ItemStack stack;
+    private final EquipmentSlotType slotType;
+    private final Multimap<Attribute, AttributeModifier> originalModifiers;
+    private Multimap<Attribute, AttributeModifier> unmodifiableModifiers;
+    @Nullable
+    private Multimap<Attribute, AttributeModifier> modifiableModifiers;
+
+    public ItemAttributeModifierEvent(ItemStack stack, EquipmentSlotType slotType, Multimap<Attribute, AttributeModifier> modifiers)
+    {
+        this.stack = stack;
+        this.slotType = slotType;
+        this.unmodifiableModifiers = this.originalModifiers = modifiers;
+    }
+
+    /**
+     * Returns an unmodifiable view of the attribute multimap. Use other methods from this event to modify the attributes map.
+     * Note that adding attributes based on existing attributes may lead to inconsistent results between the tooltip (client)
+     * and the actual attributes (server) if the listener order is different. Using {@link #getOriginalModifiers()} instead will give more consistent results.
+     */
+    public Multimap<Attribute, AttributeModifier> getModifiers()
+    {
+        return this.unmodifiableModifiers;
+    }
+
+    /**
+     * Returns the attribute map before any changes from other event listeners was made.
+     */
+    public Multimap<Attribute, AttributeModifier> getOriginalModifiers()
+    {
+        return this.originalModifiers;
+    }
+
+    /**
+     * Gets a modifiable map instance, creating it if the current map is currently unmodifiable
+     */
+    private Multimap<Attribute, AttributeModifier> getModifiableMap()
+    {
+        if (this.modifiableModifiers == null)
+        {
+            this.modifiableModifiers = HashMultimap.create(this.originalModifiers);
+            this.unmodifiableModifiers = Multimaps.unmodifiableMultimap(this.modifiableModifiers);
+        }
+        return this.modifiableModifiers;
+    }
+
+    /**
+     * Adds a new attribute modifier to the given stack.
+     * Modifier must have a consistent UUID for consistency between equipping and unequipping items.
+     * Modifier name should clearly identify the mod that added the modifier.
+     * @param attribute  Attribute
+     * @param modifier   Modifier instance.
+     * @return  True if the attribute was added, false if it was already present
+     */
+    public boolean addModifier(Attribute attribute, AttributeModifier modifier)
+    {
+        return getModifiableMap().put(attribute, modifier);
+    }
+
+    /**
+     * Removes a single modifier for the given attribute
+     * @param attribute  Attribute
+     * @param modifier   Modifier instance
+     * @return  True if an attribute was removed, false if no change
+     */
+    public boolean removeModifier(Attribute attribute, AttributeModifier modifier)
+    {
+        return getModifiableMap().remove(attribute, modifier);
+    }
+
+    /**
+     * Removes all modifiers for the given attribute
+     * @param attribute  Attribute
+     * @return  Collection of removed modifiers
+     */
+    public Collection<AttributeModifier> removeAttribute(Attribute attribute)
+    {
+        return getModifiableMap().removeAll(attribute);
+    }
+
+    /**
+     * Removes all modifiers for all attributes
+     */
+    public void clearModifiers()
+    {
+        getModifiableMap().clear();
+    }
+
+    /** Gets the slot containing this stack */
+    public EquipmentSlotType getSlotType()
+    {
+        return this.slotType;
+    }
+
+    /** Gets the item stack instance */
+    public ItemStack getItemStack()
+    {
+        return this.stack;
+    }
+}

src/test/java/net/minecraftforge/debug/item/ItemAttributeModifierTest.java

@@ -0,0 +1,55 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+package net.minecraftforge.debug.item;
+
+import net.minecraft.entity.ai.attributes.AttributeModifier;
+import net.minecraft.entity.ai.attributes.AttributeModifier.Operation;
+import net.minecraft.entity.ai.attributes.Attributes;
+import net.minecraft.inventory.EquipmentSlotType;
+import net.minecraft.item.Item;
+import net.minecraft.item.Items;
+import net.minecraftforge.event.ItemAttributeModifierEvent;
+import net.minecraftforge.eventbus.api.SubscribeEvent;
+import net.minecraftforge.fml.common.Mod;
+
+@Mod(ItemAttributeModifierTest.MOD_ID)
+@Mod.EventBusSubscriber(bus = Mod.EventBusSubscriber.Bus.FORGE, modid = ItemAttributeModifierTest.MOD_ID)
+public class ItemAttributeModifierTest
+{
+    public static final String MOD_ID = "item_modifier_test";
+    public static final boolean ENABLED = true;
+    private static final AttributeModifier MODIFIER = new AttributeModifier(MOD_ID, 10f, Operation.ADDITION);
+    
+    @SubscribeEvent
+    public static void onItemAttribute(ItemAttributeModifierEvent event)
+    {
+        if (ENABLED && event.getSlotType() == EquipmentSlotType.MAINHAND)
+        {
+            final Item item = event.getItemStack().getItem();
+            if (item == Items.APPLE)
+            {
+                event.addModifier(Attributes.ARMOR, MODIFIER);
+            }
+            else if (item == Items.GOLDEN_SWORD)
+            {
+                event.clearModifiers();
+            }
+        }
+    }
+}

src/test/resources/META-INF/mods.toml

@@ -96,5 +96,7 @@ license="LGPL v2.1"
     modId="forge_codecs_test"
 [[mods]]
     modId="render_local_player_test"
+[[mods]]
+    modId="item_modifier_test"
 [[mods]]
     modId="forge_world_type_test"